How Valency Helps in Writing Chemical Formulas

By Science Spectrum / September 21, 2026

Writing a chemical formula is not simply a matter of putting element symbols together. A correct formula must show how atoms combine in the right proportion. One of the most useful ideas for understanding this is valency. Valency tells us about the combining capacity of an atom and helps us determine how many atoms of each element are needed to form a compound.

For example, hydrogen has a valency of 1, while oxygen usually has a valency of 2. When they combine to form water, two hydrogen atoms are needed for every one oxygen atom. Therefore, the formula of water is H₂O. By understanding valency, we can write many chemical formulas systematically instead of memorizing them individually.

What Is Valency?

Valency is the combining capacity of an atom. It describes how an atom combines with other atoms to form a molecule or compound.

Atoms tend to achieve a more stable electronic arrangement. To do this, they may lose, gain, or share electrons with other atoms. The number of electrons involved in this process is related to the atom’s valency.

For example:

Hydrogen has valency 1.
Oxygen has valency 2.
Nitrogen commonly has valency 3.
Carbon commonly has valency 4.
Sodium has valency 1.
Magnesium has valency 2.
Aluminium has valency 3.
Chlorine commonly has valency 1.

Valency is especially useful when two or more elements combine to form an ionic or covalent compound.

Why Is Valency Important in Chemical Formulas?

A chemical formula tells us which elements are present in a compound and the number of atoms of each element.

Consider magnesium chloride. Magnesium has a valency of 2, while chlorine has a valency of 1. One magnesium atom needs to combine with two chlorine atoms to balance their combining capacities. Therefore, the formula is MgCl₂.

Without understanding valency, it can be difficult to know why MgCl₂ is correct instead of MgCl or Mg₂Cl.

Valency provides a simple way to determine the correct ratio of atoms.

How Valency Helps Determine the Ratio of Atoms

When elements combine, their total combining capacities must balance.

Suppose an element A has valency 2 and element B has valency 3. To balance their valencies, we need three atoms of A and two atoms of B.

The total valency of A is:

3 × 2 = 6

The total valency of B is:

2 × 3 = 6

Both sides are balanced. Therefore, the formula can be written as A₃B₂.

This idea is the foundation of the criss-cross method, which is commonly used to write formulas quickly.

The Criss-Cross Method

The criss-cross method uses the valencies of two elements to determine their subscripts.

For example, consider aluminium oxide.

Aluminium has valency 3.

Oxygen has valency 2.

Write the symbols with their valencies:

Al³⁺ O²⁻

Now cross the valency numbers:

Al₂O₃

The 2 becomes the subscript of aluminium, and the 3 becomes the subscript of oxygen.

Therefore, the formula of aluminium oxide is Al₂O₃.

The final formula must always be checked to make sure the total valencies are equal.

For Al₂O₃:

2 × 3 = 6

3 × 2 = 6

The compound is balanced.

Writing Formulas of Ionic Compounds

Valency is particularly helpful when writing formulas for ionic compounds.

Ionic compounds are generally formed when positive ions and negative ions combine. The total positive charge must balance the total negative charge.

For example, sodium has valency 1 and chlorine has valency 1.

Na⁺ and Cl⁻ combine in a 1:1 ratio:

NaCl

For magnesium oxide, magnesium has valency 2 and oxygen has valency 2:

Mg²⁺ and O²⁻

Since their valencies are equal, one magnesium atom combines with one oxygen atom:

MgO

The numbers do not need to be written as subscripts when the ratio is already 1:1.

Using Valency With Polyatomic Ions

Valency is also useful when compounds contain polyatomic ions. A polyatomic ion is a group of atoms that carries an overall charge and behaves as a unit in a compound.

Examples include:

Hydroxide, OH⁻
Nitrate, NO₃⁻
Sulfate, SO₄²⁻
Carbonate, CO₃²⁻
Ammonium, NH₄⁺

Consider calcium hydroxide.

Calcium has valency 2, while hydroxide has valency 1.

The formula is:

Ca(OH)₂

The brackets are important because two complete hydroxide groups are required.

Similarly, aluminium sulfate contains aluminium with valency 3 and sulfate with valency 2. The formula is:

Al₂(SO₄)₃

Here, two aluminium ions provide a total positive charge of 6, while three sulfate ions provide a total negative charge of 6.

Simplifying Valencies

Sometimes the valencies can be reduced to their simplest whole-number ratio.

Consider calcium oxide.

Calcium has valency 2, and oxygen has valency 2.

If we directly cross the numbers, we might initially get:

Ca₂O₂

However, both subscripts have a common factor of 2. Dividing them by 2 gives:

CaO

Therefore, the correct formula is CaO, not Ca₂O₂.

This step is important because chemical formulas are normally written using the simplest whole-number ratio of atoms or ions.

Valency and Compounds With Variable Valency

Some elements can show more than one valency. Iron is a common example. It can form compounds using valencies 2 and 3.

Iron with valency 2 is represented as Fe²⁺, while iron with valency 3 is represented as Fe³⁺.

With oxygen, iron can form:

FeO

and

Fe₂O₃

These formulas represent different compounds because the ratio between iron and oxygen atoms is different.

This is why knowing only the element’s name may not always be enough. The valency or oxidation state may also need to be considered.

Common Valencies to Remember

Learning some common valencies can make formula writing much easier.

Element or Ion	Common Valency
Hydrogen	1
Sodium	1
Potassium	1
Magnesium	2
Calcium	2
Aluminium	3
Oxygen	2
Sulfur	2
Chlorine	1
Nitrogen	3
Carbon	4

These are common values, but some elements can have more than one valency depending on the compound.

A Simple Method for Writing Chemical Formulas

When writing a formula using valency, follow these steps:

Write the symbols of the elements or ions.
Write their valencies.
Balance the valencies.
Use subscripts to show the required ratio.
Simplify the subscripts if they have a common factor.
Use brackets when more than one polyatomic ion is required.
Check that the total positive and negative charges are balanced.

For example, consider calcium chloride.

Calcium: Ca, valency 2

Chlorine: Cl, valency 1

The balanced ratio is one calcium atom to two chlorine atoms:

CaCl₂

As another example, consider aluminium oxide.

Aluminium: Al, valency 3

Oxygen: O, valency 2

The balanced ratio is:

Al₂O₃

This systematic approach reduces the need for guesswork.

Valency Is More Than a Formula-Writing Trick

Valency is not simply a shortcut used in chemistry exercises. It provides a basic way to understand why atoms combine in particular proportions.

When we write H₂O, CO₂, NaCl, MgCl₂, or Al₂O₃, the subscripts are not random numbers. They represent the ratio required for the atoms or ions to combine in a chemically balanced way.

Understanding this connection makes chemical formulas more meaningful. Instead of memorizing formulas separately, we can use the properties of the elements to work them out.

Conclusion

Valency helps us understand and write chemical formulas by showing the combining capacity of atoms and ions. By balancing valencies, we can determine the correct ratio in which elements combine. The process becomes especially useful when writing formulas for ionic compounds, compounds containing polyatomic ions, and substances involving elements with variable valency.

Learning common valencies, using the criss-cross method carefully, simplifying ratios, and checking the final formula can make formula writing much easier. More importantly, valency connects the symbols and subscripts in a chemical formula with the way atoms actually combine. Once this idea is understood, writing many chemical formulas becomes a logical process rather than a task based only on memorization.

FAQs

1. What is valency in chemistry?

Valency is the combining capacity of an atom or ion. It indicates how many electrons an atom may lose, gain, or share while forming chemical bonds. Valency helps determine how atoms combine in compounds and in what ratio they should be present. For example, hydrogen has a valency of 1, while oxygen usually has a valency of 2. Therefore, two hydrogen atoms combine with one oxygen atom to form water, H₂O. Understanding valency makes it easier to write chemical formulas because it provides a systematic way to determine the required number of atoms or ions in a compound.

2. How does valency help in writing chemical formulas?

Valency helps determine the correct ratio in which elements combine to form a compound. When two elements have different valencies, their numbers are balanced so that the total combining capacities are equal. For example, magnesium has valency 2 and chlorine has valency 1. Therefore, one magnesium atom combines with two chlorine atoms, giving MgCl₂. Similarly, aluminium has valency 3 and oxygen has valency 2, resulting in Al₂O₃. By knowing the valencies of the elements involved, we can determine the appropriate subscripts and write chemical formulas systematically rather than relying entirely on memorization.

3. What is the criss-cross method of writing chemical formulas?

The criss-cross method is a simple technique used to determine subscripts in chemical formulas from the valencies of two elements or ions. First, write the symbols and their valencies. Then, cross the valency numbers and use them as subscripts for the opposite elements. For example, aluminium has valency 3 and oxygen has valency 2. Crossing these numbers gives Al₂O₃. The resulting formula should always be checked and simplified when necessary. Although the method is convenient, it represents charge or valency balancing rather than simply moving numbers mechanically from one element to another.

4. Why is MgCl₂ the formula for magnesium chloride?

Magnesium chloride has the formula MgCl₂ because magnesium has a valency of 2, while chlorine has a valency of 1. One magnesium ion has a charge of +2, whereas each chloride ion has a charge of −1. Therefore, two chloride ions are needed to balance one magnesium ion. The total positive and negative charges become equal: +2 + (−1 × 2) = 0. This gives a ratio of one magnesium atom to two chlorine atoms. Consequently, the chemical formula is MgCl₂. Valency provides a straightforward way to understand why this particular atomic ratio is required.

5. Why is the formula of aluminium oxide Al₂O₃?

The formula of aluminium oxide is Al₂O₃ because aluminium commonly has a valency of 3, while oxygen has a valency of 2. To balance their combining capacities, two aluminium atoms provide a total valency of 6, while three oxygen atoms provide a total valency of 6. Therefore, the simplest balanced ratio is two aluminium atoms to three oxygen atoms. Using the criss-cross method also gives Al₂O₃ directly from the valencies 3 and 2. This formula represents the balanced combination of aluminium and oxygen in the compound.

6. Why do some chemical formulas need brackets?

Brackets are used when more than one complete polyatomic ion is present in a chemical formula. A polyatomic ion is a group of atoms that behaves as a single charged unit. For example, calcium has valency 2 and hydroxide has valency 1. Two hydroxide ions are required, so the formula is Ca(OH)₂. The brackets show that the subscript 2 applies to the entire OH group, not just oxygen or hydrogen. Without brackets, CaOH₂ would represent a different arrangement and would not correctly show two complete hydroxide ions.

7. Why should chemical formulas be written in the simplest ratio?

Chemical formulas are generally written using the simplest whole-number ratio of atoms or ions in a compound. For example, if valency balancing initially produces Ca₂O₂ for calcium oxide, both subscripts can be divided by 2. This gives CaO, which is the simplest ratio. Writing formulas in the simplest ratio makes them easier to understand and correctly represents the composition of the compound. The same principle applies to many ionic compounds. Therefore, after using valencies to determine the subscripts, it is important to check whether the numbers can be reduced to a smaller whole-number ratio.

8. Can an element have more than one valency?

Yes, some elements can have more than one valency. These elements can form different compounds depending on the conditions and the other element involved. Iron is a common example. It can show valencies of 2 and 3. With oxygen, iron can form FeO and Fe₂O₃. These formulas represent different compounds because iron combines with oxygen in different ratios. Therefore, when an element has variable valency, simply knowing its symbol is not enough to determine the formula. The specific valency or oxidation state involved must also be considered.

9. How do you check whether a chemical formula is correct?

A chemical formula can be checked by comparing the total positive and negative charges or the total combining capacities of the atoms or ions. For example, in MgCl₂, magnesium contributes +2 and two chloride ions contribute −2 altogether, giving a neutral compound. For Al₂O₃, two aluminium ions contribute +6 and three oxide ions contribute −6. The total charge is therefore zero. You should also check that the subscripts are in the simplest whole-number ratio and that brackets are used correctly for repeated polyatomic ions. These checks help prevent common formula-writing errors.

10. Is memorizing valencies enough to write chemical formulas?

Memorizing common valencies is useful, but understanding how they work is more important. Once you know the valencies of common elements and ions, you can use them to determine the correct ratio in a compound. For example, knowing that calcium has valency 2 and chlorine has valency 1 allows you to work out CaCl₂ logically. It is also important to understand charge balancing, simplify subscripts, recognize polyatomic ions, and identify elements with variable valency. This approach reduces dependence on memorization and helps you write unfamiliar chemical formulas more confidently and accurately.